1. Field of the Invention
The present invention relates to a magnetic shield that operates by sweeping magnetic flux lines to the periphery of the shield when cooled to a base operating temperature. Users can add the flux-sweeping magnetic-shield structure to magnetically sensitive regions of a wide range of electronic sensing applications, such as in lithographic circuit designs, in order to protect those regions from magnetic contamination or “crosstalk”.
2. Description of the Related Art
The need for improved sensing capabilities motivates low temperature detectors technologies. Operating devices at low temperatures fundamentally reduces the detectors thermal noise environment making systems capable of sensing smaller energy signals. In addition to a reduced thermal noise, lower temperatures allows superconductivity to be used in sensing applications. Superconductors offer a plethora of remarkable physical properties such as zero direct current (DC) resistance, large diamagnetic response, and despite infinite DC conductivity both low thermal conductivity and heat capacity. Further, the macroscopic quantum mechanical nature of superconductors allows for interferometry of the quantum wave function describing the superconducting condensate. Superconductors are the sensor of choice for a wide range of device applications, and their usage will continue to grow for the foreseeable future as demands continue to push the limits of technology.
Some superconducting devices include: Superconducting Quantum Interference Device (SQUIDs), Superconducting Transition Edge-Sensor (TES), Superconducting Kinetic Induction Detectors (KID), superconducting nanowire single photon detectors, Magnetic Penetration Depth Thermometers (MPTs), Josephson Junction Arrays, superconducting quantum limited parametric amplifiers, superconducting mixers, Scanning Josephson Junction (SJJ) and scanning SQUID microscopes, and superconducting quantum bits (Qubits) for quantum computing applications.
However, all the above superconductive devices could benefit from improved magnetic field control. Many of the detectors are sensitive to a very small fraction of the earth's magnetic field, making improved control essential for many applications. In many cases, along with unmatched sensitivity, superconducting devices afford sensitivity to unwanted and difficult to control physical properties (for instance magnetic field). Accordingly, control of the magnetic environment is imperative to advance many state-of-the-art superconducting systems.
Conventionally, superconducting magnetic shielding structures have been used to control magnetic field environments at low temperatures. One prior art technique includes cooling multiple concentric flexible superconducting bladders to the superconducting state. Next the bladders are expanded starting with the outermost, each expansion pulling the trapped flux lines further away creating a very low magnetic field environment in the center. Various other techniques have been attempted to improve magnetic flux expulsion in superconducting shields by creating and controlling thermal gradients in the superconductor when cooling through the transition temperature Tc. In principle, if the thermal gradient is sufficiently greater than the intrinsic spatial variation of the superconducting transition temperature Tc of the magnetic shield then the flux can be swept out. However, maintaining such thermal gradients can be challenging to implement because the thermal conductivity and heat capacity in the superconducting state can differ significantly from the normal state region, and sometimes, thermal anchoring to control the magnetic shield is limited or simply not possible. Further, the currents from biased heaters to control temperature themselves can produce unwanted sources of magnetic fields. In addition, the thermal gradients drive electrical currents which can cause flux lines to become trapped in the superconductor.
Thus, a magnetic shield implementation that can locally reduce the ambient magnetic field and sweep magnetic flux lines from sensitive components in lithographic circuit applications, for example, is needed.